The depth estimation principles of the present invention are illustratively applicable and will be described with reference to optical inspection of spiral grooves for video disc records at various manufacturing stages throughout the record mastering and replicating processes, particularly after the formation of an information track, having signal elements in the form of spaced apart depressions in the groove.
In certain high density information record/playback systems, video information is recorded as relatively short wavelength (e.g., 0.6-1.6 .mu.m) reflectivity, transmission or relief variations along the length of an information track. Illustratively, the method of recording may be of the type shown in U.S. Pat. No. 4,044,379, issued to J. B. Halter. Pursuant to the Halter method, an electromechanically driven stylus (e.g., of diamond) responsive to a video and audio signal, records the relatively short geometric variations representative of the time variations of the signal to be recorded in a metal master. After the electromechanical recording operation, the metal master has a relief pattern corresponding to that which is desired in the final record. Stampers which are used to produce production line records are made from the master and a vinyl substrate is formed, having the desired relief pattern, from the stamper.
In one illustrative format for electromechanical cutting disclosed in the Halter patent, a composite video signal is additively combined with the accompanying audio signal. In accordance with this method the accompanying audio signal is caused to frequency modulate a low frequency sound carrier over a low frequency deviation range (illustratively, 716.+-.50 KHz). In a picture modulator, the composite color video signal (including luminance signals occupying a given band of frequencies and chrominance signals appearing as sideband components of a modulated chrominance subcarrier interleaved with luminance signal components in an intermediate region of the given band) is caused to frequency modulate a high frequency picture carrier over a high frequency deviation range (illustratively, 4.3-6.3 MHz). The peak-to-peak amplitude of the sound modulator output is held at a level which is small relative to the peak-to-peak amplitude level of the picture modulator output, with an illustrative level ratio being 1:10. The respective modulated carriers are combined in a linear adder and applied to a recorder which may be a Halter electromechanical recorder controlled in response to the signal developed by the adder. The recorder is used to record the composite signal on the metal master.
The specification of the sound carrier recorded on a video disc is generally critical to the performance of the video disc system. The peak-to-peak amplitude of the sound carrier recorded on a high density information record, such as the Clemens video disc, is very small-illustratively, the sound carrier amplitude may be 85 A peak-to-peak. Deviation of the amplitude of the sound carrier from that which is specified may adversely affect the quality of the video and audio reproduction. For example, if the sound carrier is not cut deep enough the signal-to-noise ratio may be degraded or, on the other hand, if it is cut too deep sound beats may be visible during the video reproduction.
To provide high quality video and audio reproduction during disc playback a measurement of the amplitude of the sound carrier should be performed prior to the recording process to determine the depth of cut. In one prior art technique a scanning electron microscope (SEM) is used to measure the sound carrier depth. A thin stamper is made from the metal master. Samples which are prepared from the stamper are analyzed in the SEM to determine the depth of the recorded signal elements. To measure the depth of shallow (e.g., less than 200 A) and long wavelength (e.g., approximately 10 .mu.m) signal elements, such as those recorded in a Clemens disc, several measurements must be made to obtain a marginally accurate estimate of the sound signal depth. This technique is not completely satisfactory. A delay of several hours is introduced between the recording operation and the measurement; the SEM measuring process is very tedious; and the measurement is not always as accurate as desired.
In another prior art technique an optical video disc player is used to estimate the sound carrier amplitude. In accordance with this method the ratio of the detected signal at the audio frequency to that of the video frequency is measured during laser playback; thus with the knowledge of the video signal depth the audio signal depth can be determined from the ratio. This prior art technique, however, has not been satisfactory. The ratio of the video to sound is sensitive to other variables, such as focus of the laser player; therefore, it is difficult to rely on the accuracy of the results.